[media] cx18: Fix warnings introduced during cleanup

I misused the ktime API, and failed to remove some traces of the
in-kernel format conversion. Fix these, so the the driver builds
without warnings.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>